This invention relates to a communication system and, more particularly, to a telephone communication system having adaptive delta modulated (hereafter ADM) signals.
The demand for communication services has been steadily increasing. In meeting this demand, it has proven effective in some communication systems to convert signals presented to the system into encoded digital signal, and then reconvert the encoded digital signals into signals corresponding to those originally input into the system. One example of a communication system in which such transmission of encoded digital signals has proven to have particular utility is a telephone communication system. Several schemes for digitally encoding signals in a telephone system are known. Although these encoding schemes are useful for both digital and analog signals input to the telephone system, they have particular utility with continuous input, time-varying analog signals such as voice signals.
In one encoding scheme, the amplitude of a voice signal is periodically sampled and each sample converted into a digitally encoded pulse sequence or word representing a quantum of analog signal amplitudes including that of the sampled signal. This operation is called sampling and quantizing the analog signal. If the range of analog signal amplitudes represented by each quantum level or step of the quantizing operation is uniform for all analog signal amplitudes, the encoded signal is said to be linear pulse code modulated (hereafter LPCM). Each LPCM signal word may then be decoded to form an analog signal of an amplitude substantially corresponding to the amplitude of the analog signal sample encoded into the LPCM signal word. Since the input analog signal was periodically sampled, the resulting, periodic LPCM signal words may be formed into a continuous analog signal substantially corresponding to the continuous input analog signal.
In the quantizing process, the exact level of the input analog signal at the sampling instant is, as described, approximated by one of a number of discrete values or quantum levels digitally encoded as the LPCM signal. The difference between the instantaneous amplitude of the input analog signal and the quantum level actually transmitted is called quantizing error and gives rise to what is variously known as quantizing noise or quantizing distortion.
Quantizing distortion is especially objectionable and very often intolerable when the instantaneous amplitude of the input analog signal is small, but is usually of little or no significance when the instantaneous amplitude of the input analog signal is high because the low amplitude of the input signals permits a relatively low level of quantizing noise to significantly degrade the ratio of signal to noise while a higher amplitude of the input signal can tolerate greater quantizing noise within an acceptable ratio of signal to noise. It is therefore desirable to have smaller quantum levels for lower amplitudes of the input signal to achieve closer correspondence between the quantum level of the encoded signal and the actual amplitude of the input analog signal at lower amplitudes of the input signal than for higher amplitudes of the input signal. Of course the size of the quantum levels for all input signal amplitudes could be decreased, but this produces an undesirable increase in the total number of quantum levels, requiring, for example, more binary bits to represent the signal as a digitally encoded word.
The suggested non-linear redistribution of the size of the quantizing levels is called companding, a verbal contraction of the terms compression and expanding. The purpose of companding is then to reduce the quantizing impairment of the original signal without unduly increasing the total number quantizing levels by quantizing on a non-linear rather than a linear basis.
It is current practice with telephone systems to compand encoded analog signals on either a "mu-law" or an "A-law" companding scheme as described by H. Kaneko in an article entitled "A Unified Formulation of Segment Companding Laws and Synthesis of Codecs and Digital Companders," Bell System Technical Journal, September, 1970. Both these laws define segments or chords of a piecewise linear curve generally exponentially increasing for increasing levels of input analog signal amplitude. Each chord is divided into an equal number of quantization steps defining between them the intervals or quantization levels into which the analog signal will be encoded. The companding encoding scheme is then to encode each sampled analog signal amplitude into a combined sequence of two encoded signals, one representing the chord generally corresponding to the analog signal amplitude and the other representing the step along the identified chord more precisely corresponding to the analog signal amplitude. The resulting signals are then called compressed pulse code modulated signals (hereafter CPCM) or companded pulse code modulated signals. One device for so encoding input analog signals is disclosed in co-pending U.S. Pat. application Ser. No. 385,095 filed Aug. 2, 1973 in the names of Wintz, Sergo and Song. Of course, CPCM signals may also be decoded into an analog signal. One device for so decoding CPCM signals is disclosed in co-pending U.S. Pat. application Ser. No. 402,342 filed Oct. 1, 1973 in the names of Wintz and Sergo.
Still another scheme for encoding analog signals periodically samples the analog signal and compares the amplitude of the signal at each sampling instant with a signal representing the predicted amplitude of the analog signal from the immediately preceding sampling instant to form a binary-encoded signal from the comparator indicating by its one of the two possible binary states whether the instant sample of the analog signal is greater or less than the sample at the preceding instant. In general, the signal from the comparator is integrated to locally generate a signal representing the amplitude of the analog signal at the preceding sampling instant for comparison in the comparator with the instantaneous sample of the analog signal. Then, for example, if the input analog signal is greater at one sampling instant than the locally generated signal representing the amplitude of the analog signal at the immediately preceding sampling instant, the comparator provides a high logic level signal, and, if the input signal is less than the locally generated signal, the comparator provides a low logic level signal. Such binary-encoded, single-bit signals are called linear delta modulated (hereafter LDM) signals.
The effectiveness of such LDM signals in representing analog signals largely depends upon the accuracy of the locally generated signal in representing the preceding sample of the analog signal. It has been shown that the relative accuracy of the locally generated signal may be maximized by keeping the sampling rate high and the increments or quantizing steps in locally generating the signal representing the preceding analog signal relatively small to thereby provide a large number of LDM signals representing quantum levels or steps of an analog signal closely approximating the preceding signal sample so that the quantizing error in encoding an individual LDM signal will not represent a substantial excursion of the LDM signal from the actual input analog signal. Unfortunately, the sampling rates required to achieve the same quality or signal to noise ratio and dynamic range from such LDM signals in comparison to a similar signal encoded in a 7-bit mu=255 CPCM scheme is 19.6 MHz and, in an 8-bit scheme, 39.2 MHz, frequencies substantially at the limit of modern digital technology.
Nevertheless, the relative simplicity of the LDM encoding scheme makes desirable the use of this scheme in telephone equipment, particularly telephone equipment between a subscriber and a central office which generally is not now digitally encoded. Given the large number of telephone subscribers, the simplicity and thus potentially lower cost of delta modulation equipment as compared to equipment providing CPCM signals offers economic attraction for the introduction of delta modulation devices into a telephone system. However, achieving the high frequencies required for LDM signals of a quality equivalent to CPCM signals requires expensive, high-speed digital devices. In addition, the uncontrolled environment at the location of subscriber equipment makes such high-speed devices unreliable.
One solution to the high sampling frequencies required in the LDM signal encoding scheme is disclosed in co-pending U.S. Pat. application Ser. No. 482,380 filed June 24, 1974 in the name of Song. The communication system disclosed in this application has a uniform finite impulse response filter for accumulating and sampling LPCM signals converted from input LDM signals. With this digital filter it may be theoretically demonstrated that mu=255, 7-bit CPCM quality signals may be achieved from 8 MHz LDM signals instead of the 19.6 MHz LDM signals required without the filter, and mu=255, 8-bit CPCM quality signals may be achieved from 16 MHz LDM signals instead of the 39.2 MHz.
Since the effectiveness of delta modulation in representing analog signals also depends upon the size of the quantizing levels or steps in locally generating the signal representing the preceding analog signal, attempts have also been made to vary the step size in locally generating the signal representing the analog signal at the preceding sampling instant. In general similarity to the described CPCM signal companding scheme, the delta modulation step size is made smaller for ADM signals representing lower amplitude analog signals than for ADM signals representing higher amplitude analog signals. This technique is usually called adaptive delta modulation. Signals encoded in such a scheme are then called adaptive delta modulated (hereafter ADM)signals.
Several proposals for implementing ADM signal encoding schemes are reviewed in an article by H. R. Schindler, "Delta modulation," IEEE spectrum, October, 1970. As described in this article, one early proposal doubles the quantization step size of an integrator which locally generates an analog signal which is compared with the input signal to form the ADM signals in response to two consecutive ADM signals of identical logic state and halves the step size in response to two consecutive ADM signals of alternate logic state. This proposal then uses patterns of identical or alternating logic states of two consecutive ADM signals to control integration step sizes. Control of integration step size, being an analog procedure, is difficult to stabilize.
Other proposals continuously monitor an input analog signal to generate, in addition to ADM signals, a signal describing the quantization step size represented by the ADM signals. Both signals are then transmitted to a detector which continuously applies the step describing signal to the successive ADM signals to generate an analog signal on which the increment represented by each ADM signal forming the analog signal is controlled by the separately transmitted signal. Such double signal transmission substantially defeats the attractive simplicity of the ADM signal encoding scheme.
U.S. Pat. No. 3,500,441 issued in the name of Brolin discloses another device for creating ADM signals from input analog signals. This device is similar to that just described in that the size of integration steps represented by each ADM signal determined in direct response to changes in the input analog signal and the determined integration step size represented by a signal distinct frm the ADM signal. However, in this device the distinct step signal is digitally encoded and time division multiplexed with the ADM signal for transmission to a decoder. The decoder then demultiplexes the two signals to control integration of the ADM signal with the distinct, digital step signal. Again, such double signal transmission substantially defeats the attractive simplicity of the ADM signal encoding scheme.
Another proposal for an ADM signal encoding scheme attempted to avoid the undesirable double signal transmission with a return to the concept of the above described early proposal in which the successive ADM signals themselves carry information indicating the analog increment represented by each ADM signal while, at the same time, retaining continuous control of the analog increment represented by each ADM signal. In this proposal the successive ADM signals are rectified into an analog signal which controls the local generation of a second analog signal from each of the successive ADM signals. The second analog signal is then compared with the input analog signal for forming the ADM signals. But, as with the early proposal, the control of one analog signal with another analog signal is difficult to stabilize for consistent control and matching of the characteristics of sending and receiving paths of the system.
Still another proposal has a sequence detector which detects one specific pattern of four consecutive ADM signals of identical logic state and, in response to the detected pattern, generates an analog signal potential which is a successive integral multiple of a constant determined by the logic state of the ADM signals forming the pattern. The analog potential then controls the width of a generated pulse which is therefore an analog time signal. The pulse then controls the duty cycle of a single current source driving an integrator which forms another analog signal which is compared with the input signal for forming the ADM signals. The analog signal increments represented by these ADM signals are theoretically logarithmically related but this proposal again teaches the difficult to stabilize analog control of an analog signal.
Other patterns of ADM signals for determining the increment of an analog signal represented by each ADM signal are discussed in "Adaptive Delta Modulation With a One Bit Memory," N. S. Jayant, Bell System Technical Journal, March, 1970. One specific pattern, later described by the 7-7-4-3 rule, was empirically developed by T. H. Daugherty as reported in "Digitally Companded Delta Modulation for Voice Transmission," 1970 IEEE Circuit Theory Symposium.
U.S. Pat. No. 3,652,957 issued in the name of Goodman discloses another device for implementing an ADM signal encoding scheme in which analog signals are first converted at a high sampling rate into high-speed LDM signals and the high-speed LDM signals then converted in a counter into LPCM signals. The LPCM signals in the counter are compared in a comparator with signals from an accumulator representing the preceding LPCM signal. The comparator periodically provides lower rate. ADM signals which increment the accumulator. The accumulator is additionally responsive to digital logic which controls the number and sign of the accumulator increments for each ADM signal. Several embodiments of the digital logic are suggested. In one embodiment the digital logic is a read-only memory which performs a known table-look-up function in response to detected patterns of successive ADM signals. In another embodiment the digital logic performs a calculating function in which each successive accummulator counting increment is an arithmetic function, that is, a constant increment added to or mulitple of the previous increment. Although this device avoids analog control of an analog signal with digital logic and an accumulator, it requires high speed LDM signal encoding devices to provide high quality ADM signals.